(i) Field of the Invention
The present invention relates to the field of so-called xe2x80x9cgas-insulatedxe2x80x9d high-voltage current transmission lines (the name GIL is often found in the literature).
(ii) Description of the Related Art
In this field, there is a clear tendency to restrict the existence and installation of overhead high-voltage lines in preference to so-called xe2x80x9cgas-insulatedxe2x80x9d lines or else solid-insulation lines which it is possible to bury.
It will be recalled here that the typical structure of such current transmission lines or cables comprises an outer tubular sheath and at least one conductor which is internal to the sheath and which is coaxial with this sheath, the internal conductor or conductors of the cable being insulatingly supported on the internal structure of the outer sheath, the space lying between internal conductor(s) and outer sheath being filled with a insulating gas mixture.
Quite recent and extensive literature thus mentions, as insulating gases, SF6 or else gas mixtures based on SF6 (of which SF6 the electrical insulation properties over a wide temperature range are well known), as well as the possible problems of recovering, purifying and recycling the SF6 used in such electrical engineering applications.
By way of illustration, the following documents will be mentioned here: DE-19,623,723, DE-3,122,886, DE-19,503,227, WO-98/23363, US-2,415,763, EP-820,801 and US-4,705,914.
The literature also mentions, as insulating gas mixtures, mixtures based on CO2, CF4 or SO2.
What is immediately apparent from reading all this literature is the fact that, among the main difficulties in this technology of gas-insulated lines using SF6, one finds, on the one hand, the fact that losses due to SF6 leakage must comply with the extremely restrictive international standards (in practice, they must not exceed 1% per year of the SF6 introduced) and, on the other hand, the difficulties associated with controlling the handling of the gases involved and the filling phase in order to meet the required specifications, especially in terms of residual content of oxygen and of water vapor in the insulating atmosphere internal to the line.
The WO document mentioned above proposes, for example, a method for producing a mixture containing SF6 in two stages: firstly the manufacture of an inhomogeneous premixture, then the storage of this premixture in a buffer tank, if necessary it having been passed beforehand through a static mixer, the buffer tank then being continuously repressurized by the fact that some of the mixture coming from the tank is compressed and recycled into the buffer tank.
It is therefore recognized that there is a real need to have a simple and reliable process for producing such insulating gas mixtures based especially on SF6, as well as improved methods of filling such gas-insulated electric current transmission lines with gas that allow the required specifications to be met.
One of the objectives of the present invention is to propose a solution to the technical problems listed above.
To do this, the invention relates to a method for filling so-called gas-insulated electric current transmission lines with gas using an insulating gas mixture, which line comprises an outer tubular sheath and at least one conductor internal to the sheath and coaxial with this sheath, which method comprises the implementation of the following steps, applied to the space lying between the internal conductor or conductors and the outer sheath:
a) application to said space of at least one purge cycle comprising:
a pumping step, down to a first vacuum level;
a purge step by introducing a purge gas;
b) application to said space of a filling cycle comprising:
a pumping step, down to a second vacuum level;
a filling step using said insulating gas.
The gas-filling method according to the invention may moreover adopt one or more of the following characteristics:
the first vacuum level is less than 50 mbar, and preferably lying within a range going from 5 to 20 millibars;
the second vacuum level is less than 100 mbar, and preferably lying within a range going from 0.1 to 20 millibars;
the atmosphere internal to the line as obtained after the purge cycle or cycles is analyzed in terms of at least one gaseous component;
the atmosphere internal to the line as obtained after the filling step of the filling cycle is analyzed in terms of at least one gaseous component;
the pressure in the line is measured after the pumping step of the filling cycle.
The invention also relates to a process for manufacturing so-called gas-insulated electric current transmission lines, lines of the type comprising an outer tubular sheath and at least one conductor internal to the sheath and coaxial with this sheath, the process comprising a phase of filling the space lying between the internal conductor or conductors and the outer sheath using an insulating gas mixture and carrying out the filling phase by applying the method for filling electric current transmission lines with gas as described above.
The invention also relates to a process for producing a homogeneous gas mixture containing SF6 and a carrier gas, which process comprises the implementation of the following measures:
a first line for supplying a primary gas containing SF6 is used;
a second line for supplying a carrier gas is used;
one of the two supply lines is equipped with a flow measurement device while the other line is then equipped with a flow-regulating device;
the two supply lines are connected at their downstream part to a line for delivering the mixture to a consumer point;
using a pressure-regulating device located in the delivery line, the pressure of the mixture delivered by the delivery line downstream of the points of connection of the supply lines is regulated.
The process for producing a homogeneous gas mixture containing SF6 and a carrier gas according to the invention may moreover include one or more of the following characteristics:
a dynamic mixing device, located in the delivery line upstream of said pressure-regulating device, is used;
a dynamic mixing device, located in the delivery line between said pressure-regulating device and the consumer point, is used;
the dynamic mixer consists of a pipe having, over all or part of its length, a nonrectilinear structure of turns, the pipe being provided with a gas inlet connected to an upstream part of the delivery line and with a gas outlet connected to a downstream part of the delivery line;
pressure regulation is achieved by the presence, in the delivery line, of a device chosen from the group consisting of a control valve, a backpressure regulator, a calibrated orifice and a flow regulator.
The notion of xe2x80x9cdynamicxe2x80x9d mixing according to the invention should be understood to mean the fact of producing, in the mixer, a minimum pressure drop, without a stagnation region or region of zero gas velocity.
Further characteristics and advantages will merge from the following description, given solely by way of example, and with reference to the appended drawings.